The invention relates to semiconductor materials and devices characterization or evaluation, and more particularly to the electrical and optical characterization of light emitting diode (LED) device structures. The invention will be described with particular reference to the characterization of quantum well-based LED structures. However, the invention is not so limited, but will also find application in optical and optoelectronic evaluation of p/n junctions, semiconductor laser structures, and the like.
The prior art discloses semiconductor characterization using a very broad range of experimental techniques. Semiconductor materials and devices are commonly characterized or evaluated using x-ray diffractometry, photoluminescence, cathodoluminescence, and electroluminescence, among many other techniques. In the case of optoelectronic devices which convert electrical energy to optical energy and/or vise versa, methods which excite luminescence in the material are particularly useful. In photoluminescence, excess carriers (excess electron-hole pairs) are photoexcited by exposure to a sufficiently intense light source, and the luminescence emitted as these photoexcited carriers recombine is measured. The luminescence can be measured spectroscopically and/or as a function of time after the light source is turned off. Cathodoluminescence is similar to photoluminescence except that the excess carriers are generated by exposure to an electron beam rather than by exposure to light.
For evaluating a light emitting diode (LED) device structure, the electroluminescence behavior is of greatest interest, as the finished LED device functions through electroluminescence. Electroluminescence is similar to photoluminescence and cathodoluminescence, except that in electroluminescence the excess carriers are electrically injected. In the case of an LED, the electrical injection of carriers into the optically active p/n junction region is achieved by forward biasing the p/n junction. However, electroluminescence is not equivalent to photoluminescence, because the electroluminescence behavior of a sample is determined by a number of factors, such as the optical properties of the optically active layers, the electrical transport properties (e.g., conductivity) of the p-type and n-type regions, and the properties of the electrical contacts through which the electrical biasing is applied. Some of these factors, particularly those relating to transport, can produce different effects on the electroluminescence versus the photoluminescence. It is to be appreciated that in photoluminescence, both the excess conduction electrons and the excess holes are typically injected into the same side of the junction, whereas in electroluminescence the injection of electrons and holes are on opposite sides of the junction.
An important class of LED""s are epitaxially grown double heterostructure-based LED""s (DH-LED""s). In these devices, the simple doping junction of the standard p/n diode LED is replaced by an active region containing luminescent material, and with an energy gap less than that of the surrounding p and n type materials. The active region is preferably sandwiched between the p-type and n-type regions of the DH-LED. Light emission in a DH-LED is through the radiative recombination of electrically injected excess carriers inside the active layer. The active layer of a DH-LED defines a potential well. If the dimension of the active layer is less than about 10 nm, then the double heterostructure is called a quantum well. Multiple quantum wells can exist in the active layer of a heterostructure LED.
The active region of a DH-LED serves, in addition to physically hosting the luminescent material, as a carrier confinement region that confines carriers inside the active layer or quantum wells. If an electron-hole pair exists inside a potential well, the likelihood of recombination increases as the width of the well decreases. This is simply because the electron is physically closer to the hole in a narrow potential well than in a wider potential well.
The electroluminescence of DH-LED""s and quantum well-based LED""s is further complicated by the additional structural complexity. The electroluminescence can be affected by factors such as the effectiveness of the carrier confinement, interfacial defects, impurities at the quantum well boundaries or inside the quantum wells, the relative confinement of conduction electrons versus holes (typically determined by the conduction band and valence band offsets at the interface between the quantum well and the barrier material), crystalline quality of the quantum wells, atomic interdiffusion at the quantum well interfaces, and the like. It will again be appreciated that these effects can be different for electroluminescence versus photoluminescence.
Commercial LED wafers are typically tested at the wafer level using photoluminescence. However, it is generally known to the art that high photoluminescence efficiency is a necessary but not a sufficient test of an LED wafer. A wafer that exhibits poor active layer photoluminescence properties will usually also exhibit poor electroluminescence behavior, translating into poor LED""s fabricated therefrom. However, a wafer with high photoluminescence efficiency may or may not produce high electroluminescence efficiency and hence good LED""s, because of differences between the electroluminescence and photoluminescence processes as discussed above. Thus, there remains an unfulfilled need for improved screening of LED wafers at the wafer level.
The prior art also does not teach effective means for separating out the various components of the electroluminescence signal. Poor electroluminescence or LED behavior can result from failure at any layer of the LED structure, or from problems introduced during LED fabrication. The prior art teaches generating a matrix of varying sample growth conditions and fabrication steps and analyzing the matrix, e.g. by fabricating LED""s therefrom, in the hope of correlating the matrix parameters with changes in the LED behavior or the electroluminescence. This approach has several disadvantages. First, it is expensive in terms of personnel time, equipment load, and source materials. Second, it is highly subjective. Misleading results can easily be obtained if elements of the sample matrix include unknown variations, e.g. differences in doping level between samples for a layer which has the same nominal doping level for all the samples of the matrix. Even if an unintended matrix variation is recognized, e.g. through doping concentration measurements, it still can be difficult or even impossible to correct the data therefor.
In view of these disadvantages, it would be useful to have an improved characterization method that preferably is performed at the wafer level and more closely resembles the physical mechanisms of electroluminescence and LED operation, and that has the ability to independently evaluate for a single sample the relative contributions or effects on the electroluminescence characteristics of the various sample regions such as the active region including the quantum well or wells, the p-type material region, the n-type material region, and the electrical contacts.
The present invention contemplates such an improved characterization or evaluation method and apparatus.
In accordance with one aspect of the present invention, an apparatus for evaluating an associated semiconductor sample is disclosed. The associated sample has a first electrically distinct region and a second electrically distinct region, and further has a junction region disposed therebetween. The evaluation apparatus includes a stage for mounting the semiconductor sample. A first laser has a wavelength tuned to photogenerate carriers in the first electrically distinct region. An electrical biasing means is provided for impressing an electrical field whereby at least some photoexcited carriers are influenced to drift toward the junction region. The photoexcited carriers are holes from the p-side and electrons from the n-side. In this manner, instead of injecting electron-hole pairs from one side through thermal diffusion, electrons and holes are injected from different sides as they would be in an actual LED. An optical detector is provided, whereby luminescence generated by recombination of the photoexcited carriers in the junction region is detected.
Preferably, the apparatus includes a translation means for relatively translating the laser and the sample whereby the laser beam is scanned across the sample. A second laser is preferably disposed on the opposite side of the sample with respect to the first laser. The second laser has a wavelength tuned to photogenerate carriers into the second electrically distinct region. Preferably, the first laser has a wavelength tuned to a first energy approximately corresponding to the energy band gap of a material comprising the first electrically distinct region, while the second laser has a wavelength tuned to a second energy approximately corresponding to the energy band gap of a material comprising the second electrically distinct region. Optionally, the two wavelengths can be the same, i.e. the same laser beam is split to serve as both the first laser and the second laser.
In one application, the associated sample has at least one potential well in the junction region. The optical detector preferably has a detection wavelength range which essentially includes the active layer luminescence. In a more specific application, the first region of the associated sample includes n-type gallium nitride, the second region of the associated sample includes p-type gallium nitride, and the active layer of the associated sample includes an alloy of indium gallium nitride. In this case, the first laser and the second laser preferably have wavelengths less than 365 nm to provide adequate absorption by the semiconductor. Preferably, at least one of the group including the first laser and the second laser is a tunable wavelength laser.
In accordance with another aspect of the present invention, a method for characterizing an associated semiconductor sample is disclosed. The associated sample has a first electrically distinct region and a second electrically distinct region, and further has a junction region disposed therebetween. The characterization method includes the steps of optically generating carriers in the first electrically distinct region, generating an externally applied drift field in the first region that effectuates a drifting of the optically generated carriers in the first electrically distinct region toward the junction region, and measuring the optical radiation generated by radiative recombination of the optically generated carriers in the junction region.
Preferably, the characterization method also includes optically generating carriers in the second electrically distinct region, and generating an externally applied drift field in the second region that effectuates a drifting of the optically generated carriers in the second electrically distinct region toward the junction region. Typically, the step of generating an externally applied drift field in the first region and the step of generating an externally applied drift field in the second region are performed together by applying a voltage between an electric contact that electrically contacts the first electrically distinct region and an electric contact that electrically contacts the second electrically distinct region.
In the step of generating an externally applied drift field in the first region, an electric drift field described by a field vector E is generated. Preferably, in the step of optically generating carriers in the first electrically distinct region, the optically generated carriers are substantially generated within a distance d=xcexcxcfx84|E| of the junction region, where xcexc is the drift mobility of the optically generated carriers in the first material, and xcfx84 is the lifetime of the optically generated carriers in the first material. Under these conditions, the fraction of the optically generated carriers which enter the junction region is approximately 1/e.
The method preferably further includes estimating quantitatively the volume recombination rate in the junction region based on the step of measuring the optical radiation generated by radiative recombination of the optically generated carriers in the junction region; estimating quantitatively the volume density of optically generated carriers in the first electrically distinct region; and estimating quantitatively the electroluminescence efficiency based upon the volume recombination rate and the volume density of optically generated carriers.
In the above method, the magnitude of the drift field produced in the step of generating an externally applied drift field in the first region is preferably sufficiently low such that the number of carriers electrically generated is negligible compared to the optically generated carriers.
In accordance with yet another aspect of the present invention, A method for characterizing a light emitting diode (LED) structure sample is disclosed. The sample has an n-type region and a p-type region with a junction region disposed therebetween. Carriers are optically generated in the n-type region by light impingement thereon. Carriers are optically generated in the p-type region by light impingement thereon. The optical radiation generated by radiative recombination of the optically generated carriers in the junction region is measured.
Preferably, the method further includes electrically biasing the junction and to effectuate a drifting of the optically generated carriers toward the junction region.
Preferably, the method further includes optically chopping the impinging light with an optical chopper, detecting the optical radiation with an optical detector, and measuring the optical detector signal at the optical chopping frequency using a lock-in amplifier that is in operative communication with the optical chopper and the optical detector.
Preferably, the method further includes repeating the generating, biasing, and measuring steps at a plurality of wavelengths of the at least one optical source, and estimating transport properties of the at least one region therefrom.
Preferably, the method further includes repeating the generating, biasing, and measuring steps at a plurality of intensities of the at least one optical source, and estimating the effects of high injection levels from the measuring.
One advantage of the present invention is that it permits separately probing the effects of transport in the p-type and n-type regions, artifacts due to the electrical contacts, and properties intrinsic to the active region.
Another advantage of the present invention is that it permits spatial profiling of the LED heterostructure across the wafer.
Another advantage of the present invention is that it permits depth-dependent profiling into both the n-side and the p-side of the LED structure.
Yet another advantage of the present invention is that it facilitates photoexcited electroluminescence whereby simultaneous excitation from the front and the rear of the wafer is performed.
Still yet another advantage of the present invention is that it provides a wafer level characterization method that is closer to the physical behavior of an operating LED versus prior art wafer level characterization methods.
Still further advantages and benefits of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description.